1. Field of the Invention
The present invention relates to an automatic focus device that enables the focussing lens included in the shooting lens (photographic lens) to immediately follow a moving subject.
2. Description of Related Art
An automatic focus device has been known from that conducts focus adjustment by automatically moving the focussing lens included in the shooting lens relative to a moving subject (see, for example, Japanese Patent Publication No. 63-148218). With this type of conventional automatic focus device, the defocus amount and velocity of the image-forming plane of focus state detection light rays that have passed through the shooting lens (hereafter referred to as the subject image-forming plane) are repeatedly computed, and driving of the focussing lens is controlled on the basis of the computation results. The defocus amount refers to the relative amount, of deviation and direction of deviation between the subject image-forming plane and the film plane (predicted focal plane).
However, when the subject is not moving at a fixed velocity, it is impossible to correctly drive the focussing lens to the focus position even if the focussing lens is driven on the basis of the previous computation results. In addition, even when the subject moves at a constant velocity, the subject image-forming plane does not necessarily move at a constant velocity. In general, the velocity of the subject image-forming plane (hereinafter called the image plane velocity) becomes faster the nearer the subject comes to the automatic focus device. Conversely, the image plane velocity becomes slower the farther the subject is from the automatic focus device. Accordingly, accurate predictions are not possible when the subject position is predicted simply on the basis of the previous computation results.
Calling v the constant velocity with which the subject is approaching the automatic focus device, and h the distance where the subject is closest (hereafter referred to as the closest position), the position y of the subject image-forming plane (hereafter referred to as the image plane position) is given by formula 1, wherein f is the focus length of the shooting lens and t is time. ##EQU1##
FIG. 2 is a graph illustrating the relationship given by formula (1) with the time t (in seconds) on the horizontal axis and the image plane position y (in mm) on the vertical axis. The curve in FIG. 2 shows the example wherein the focal length f of the shooting lens is 400 mm, the movement velocity v of the subject is 80 km/h, and the closest position h is 10 m. The height of the peak in the curve shown in the drawing and the slope of the curve will change as the movement velocity v of the subject and the closest position h vary.
As shown in FIG. 2, the curve given by formula (1) reaches the peak at time t=0, and the relationship between the image plane position y and the time t is non-linear. In particular, the change in the image plane position y relative to time t is larger near the peak point in the graph. However, conventional automatic focus devices predict the subject position by approximating each point on the curve in FIG. 2 as being linear, and consequently, when the subject position is predicted near the peak point where the change in slope is large in the curve in FIG. 2, the amount of deviation from the actual subject position becomes large.
It is thus impossible to predict with a high degree of precision future image plane positions even when the positions are predicted using linear approximations on the basis of past image plane velocities or image plane positions. Consequently, an automatic focus device has been proposed that predicts the subject position on the basis of the acceleration of the subject image plane (hereafter referred to as the image plane acceleration). With such a device, a future subject position is predicted using the quadratic equation shown below as formula (2), where the coefficients a, b and c in this equation are set on the basis of the image plane positions detected at three times, two previous times and the present time, and the detection time of each. EQU y=a.multidot.t.sup.2 +b.multidot.t+c (2)
With formula (2), the prediction is equivalent to predicting the subject position on the basis of the image plane acceleration because the image plane position at three different times are taken into consideration. This is because the image plane velocity can be found by integrating the image plane acceleration relative to time, and the image plane position can be found by integrating again relative to time. Consequently, the relationship between the image plane acceleration and the image plane position is a quadratic equation such as shown in formula (2).
If the image plane position is predicted on the basis of formula (2), a prediction with a higher degree of precision is possible than can be achieved by simply predicting the image plane position through linear approximations of the image plane velocity. However, because formula (2) describes a parabola in contrast to the curve shown in FIG. 2 as the relationship between time t and image plane position, the error becomes larger for the following reasons.
As noted above, the relationship between time t and image-forming plane position y is as shown by formula (1), but the focal distance f in the denominator of formula (1) can be ignored because the term is small in comparison with the other terms, so formula (1) can effectively be rewritten as in formula (3). ##EQU2##
Differentiating formula (3) relative to time t, the image plane velocity and image plane acceleration are given by formulae (4) and (5), respectively. ##EQU3##
As shown by formulae (4) and (5), the image plane velocity becomes a higher order function than the image plane position, and the image plane acceleration becomes a still higher order function. In addition, as the time t approaches zero, in other words, as the peak in the curve in FIG. 2 is approached, the image plane acceleration changes greatly in a non-linear way. Accordingly, it is impossible to approximate this kind of curve with a high degree of precision using the parabolic approximation of formula (2). This is because a parabola assumes that the acceleration is constant and that the velocity changes with a fixed ratio, so that the error near the peak of the curve in FIG. 2 will be particularly large.